This invention relates to methods of producing a reflection type mask blank and a reflection type mask for exposure for use in semiconductor pattern transfer and the like, and a method of producing a semiconductor device.
Recently, in the semiconductor industry, an EUV lithography, which is an exposure technique using extreme ultra violet (Extreme Ultra Violet, EUV) light, appears promising following the miniaturization of a semiconductor device. It is noted here that the EUV light means light of a wavelength band within a soft X-ray region or a vacuum ultraviolet region, specifically, light having a wavelength of about 0.2-100 nm. As a mask used in the EUV lithography, proposal is made of a reflection type mask as disclosed in JP-A No. H8-213303.
The reflection type mask mentioned above has a structure comprising a multilayer reflection film formed on a substrate for reflecting the EUV light and an absorber layer formed as a pattern on the multilayer reflection film for absorbing the EUV light. In an exposure apparatus (pattern transfer apparatus) to which the reflection type mask mentioned above is mounted, exposure light incident to the reflection type mask is absorbed at a part where the absorber layer pattern is present and is reflected by the multilayer reflection film at another part where the absorber layer pattern is not present to form an optical image which is transferred through a reflection optical system onto a semiconductor substrate (a silicon wafer with a resist).
As the multilayer reflection film mentioned above, use is generally made of a multilayer film in which a material having a relatively high refractive index and a material having a relatively low refractive index are alternately laminated by the thickness on the order of several nanometers. For example, a multilayer film obtained by alternately laminating Si films and Mo films is known as a film having high reflectance for the EUV light of 13-14 nm. In the reflection type mask using the multilayer reflection film mentioned above, the film density of each layer of the multilayer film must be increased in order to obtain high reflectance with respect to light having a short wavelength. As a consequence, the multilayer reflective film inevitably has a high compression stress.
According to the study of the present inventor, however, it has been found out that, after the multilayer reflection film is deposited, the stress of the film is changed with the lapse of time by about 30 MPa in a tensile direction due to thermal factors, even if the film is used or stored in a normal environment. Such change in stress intermittently occurs, for example, in a period of one year.
It has been found out that, in a production process of a reflection type mask blank after deposition of the multilayer reflection film, the change in stress of the multilayer reflection film also occurs due to thermal factors in a cleaning step, a baking step after deposition of a resist film, and the like.
Presumably, the change in stress results from very slight mixing at an interface between respective layers forming the multilayer reflection film. Such change is of a level which can not be detected by measurement of a period length by the X-ray diffraction analysis but causes the peak wavelength of the reflectance of the multilayer reflection film (i.e., the wavelength at which the reflectance of the multilayer reflection film has a peak value (maximum value)) to be changed by the level of 0.01 nm. Since the EUV light has a very short wavelength, the change in state of the multilayer reflection film very sensitively affect the wavelength characteristic and the reflection characteristic thereof.
In the EUV lithography, the light in a specific narrow wavelength band is used so that the influence of wavelength shift is great. The shift in peak wavelength of the reflectance causes mismatching with a mirror of the exposure apparatus used upon pattern transfer. Therefore, the peak wavelength must accurately be controlled. Furthermore, the shift in peak wavelength causes a decrease in reflectance of the multilayer reflection film. Thus, the change with time in stress of the multilayer reflection film poses various problems in practical use of the mask, for example, causes the change in flatness of a substrate.
In existing reflection type masks, however, no consideration has been made of the above-mentioned problems due to the change with time in stress of the multilayer reflection film after the multilayer reflection film is formed and no means for solving the problems has been found out.